1. Field of the Invention
The present invention relates to a multiplex transmission system which is used for automobile electric systems of a multiplex transmission type to transmit data in a multiplex mode.
2. Description of the Related Art
Conventionally, in the multiplex transmission system of this type, a plurality of multiplex transmission devices (hereinafter called nodes) are connected via a transmission line consisting of a twisted pair etc., and these nodes and the transmission line constitute a multiplex communication network.
The nodes are broadly classified into two categories according to the method for fulfilling the communication control function: a node requiring software to fulfill the multiplex communication function (hereinafter called a basic node) and a node requiring no software to fulfill the multiplex communication function (hereinafter called an I/O node).
The basic node can optionally set the data transmission timing by using the control function of a microcomputer. Therefore, the basic node can spontaneously perform cyclic data transmission. The basic node used for automobile electric system includes an instrument panel node, a cowl node, a seat node, and other nodes requiring an application control function. The instrument panel node, being located, for example, near the meters, controls the function of the meter. The cowl node, being located, for example, at the cowl position, controls the timer function and the alarm function. The seat node carries out the operation control of memory for storing, for example, the seat position of power seat.
The I/O node fulfills the data input/output control function by using hardware. The I/O node requires no microcomputer when no application control function is needed, which offers an advantage of decreased node cost. On the other hand, the I/O node cannot optionally set the data transmission timing, unlike the basic node, because it has no microcomputer. Therefore, the I/O node sends out data to the transmission line, for example, when the logic level of input signal, which is inputted from the outside to an input circuit, changes from a high level to a low level or from a low level to a high level. The I/O node Used for automobile electric systems includes a door node and other nodes requiring no application control function. The door node transmits and receives signals relating to the drive of the auxiliary driving equipment (door lock motor) arranged on the door.
With the conventional multiplex transmission system, the basic node fulfills the multiplex communication function by using a communication control program in the microcomputer and a communication control circuit, so that a frame, which is a communication data block, is transmitted and received.
The multiplex transmission system of this type sometimes produces a data transmission error, though the probability is very low. To solve this problem, there is a method for enhancing reliability of data transmission, which is provided by the cyclic transmission of data, as one of fail-safe measures against the data transmission error. With this method, even if a transmission error occurs at the node on the signal receiving side with a very low probability, the data is returned to the correct condition by the frame sent next, which attains fail-safe. For example, in the cowl node, the cyclic transmission which is performed for the necessity of the fail-safe measures can be achieved by the control function of the microcomputer. For example, for the frame for the door node, the data of driving signal of the door lock motor, which is the auxiliary driving equipment of door node, is allocated to the data area in the frame transmitted cyclically.
The I/O node, having no microcomputer, fulfills the input/output control function and the multiplex communication control function by using a communication controller (hereinafter called an I/O controller). For this reason, the I/O node cannot perform cyclic transmission spontaneously, unlike the basic node. In the conventional I/O node, therefore, an oscillation circuit is connected to the input circuit so that the periodic pulse signals from the oscillation circuit are inputted to the input circuit. This periodically changes the input signal inputted to the input circuit, thus the I/O controller cyclically performing frame transmission.
In the multiplex transmission system of this type, however, a space for mounting the oscillation circuit must be provided in the I/O node because the I/O node has the oscillation circuit to perform cyclic data transmission. Therefore, the conventional multiplex transmission system has problems of large-scale system and increased cost. In addition, the I/O node cannot detect a fault if the signal transmitting/receiving circuit etc. becomes faulty, because it has no microcomputer.